The present invention relates to liquid ejection heads used in liquid jetting devices such as ink jet recording heads used in image recording apparatuses such as a printer, colorant ejection heads used for manufacture of color filters of a liquid crystal display etc., electrode material ejection heads used for formation of electrodes of an organic EL (electroluminescence) display, an FED (field emission display), etc., and bio-organic material ejection heads used for manufacture of bio-chips (biochemical devices).
A liquid ejection head is known that is equipped with: an actuator unit in which a piezoelectric element group is joined to the surface of a metal fixing plate; a case that houses the actuator unit; and a cavity unit in which pressure generating chambers and nozzle orifices are formed and that is joined to the front end of the case.
The above case is made of a synthetic resin such as an epoxy resin to facilitate shaping process and mass-production. An chamber for accommodation and fixing of the actuator unit is formed in the case. The chamber is provided for each actuator unit if there exist a plurality of actuator units. Therefore, in a liquid ejection head having a plurality of actuator units, a partition that was molded integrally with the case is provided between adjoining chambers. Each actuator unit is fixed in the associated chamber by bonding it to a partition wall of the case in a state that the piezoelectric element group is divided into piezoelectric elements with the same pitch as the arrangement pitch of the nozzle orifices are positioned with respect to respective pressure generating chambers, more specifically, a vibration plate that seals the pressure generating chambers.
The cavity unit is formed by bonding together a nozzle plate through which a plurality of nozzle orifices are formed, a chamber formation plate that forms pressure generating chambers corresponding to the respective nozzle orifices, and a vibration plate that seals the chamber formation plate and to which the free end portions of the piezoelectric elements are joined. The nozzle plate and the chamber formation plate of the cavity unit are made of silicone or a metal such as stainless steel because they should be highly rigid to keep the liquid droplet jetting characteristic constant. On the other hand, the vibration plate is a composite member having a double-layer structure in which a resin elastic film or metal foil is laminated on a metal support plate.
Temperature variation causes a large difference between expansion or contraction lengths of the case and the cavity unit because of a large difference between the linear expansion coefficients of a synthetic resin as the material of the case and a metal as the material of the cavity unit. As a result, peeling may occur between the case and the cavity unit, in particular, at end portions of the bonding area and both ends of each pressure generating chamber, even when a case as small as about 2 μm, for example, expands or contracts.
When the liquid ejection head is placed in a high-humidity environment or, conversely, in a dry environment, the above-mentioned case made of a synthetic resin is deformed through moisture absorption or release. Such deformation may cause peeling of the case and the cavity unit off each other. In case where the case is deformed, there is an anxiety that the actuator unit fixed to the case, that is, the piezoelectric elements are inclined. In such a case, stress acts on the bonding boundaries between the piezoelectric elements and the vibration plate and the piezoelectric elements become prone to peel off the vibration plate. If the piezoelectric elements are driven in this state, the ejection of liquid droplets may receive adverse effects such as generation of crosstalk.
In view of the above, an attempt to employ a metal case has been made. However, this is associated with a problem that it is more difficult to work a metal case into a complex shape than a resin case and hence the production efficiency cannot be increased easily.
For example, Japanese Patent Publication No. 9-99557. (cf., page 4 and FIG. 1) discloses a configuration in which a buffer member made of metal or thermosetting resin is interposed between the case (frame) and the cavity unit (ink chamber formation member) to reduce the stress generated between the case and the cavity unit resulting from the difference between the linear expansion coefficients thereof.
However, the deformation (expansion or shrinkage) of the case cannot be avoided securely by merely providing the buffer member between the case and the cavity unit. Accordingly, the disadvantageous situation resulting from the case deformation cannot be suppressed securely.